deps: Pin Pydantic to <2.9.0

A loose dependency on Pydantic (>=2.0) could pull in incompatible
newer versions, leading to a critical startup error. This manifested
as a "TypeError: bool is not iterable".

This commit pins the requirement to `<2.9.0` to ensure compatibility
with the current Gradio version. Version 2.8.x was the latest stable
series at the time of the Gradio 4.44.1 release.

Refs: gradio/gradio#10649, gradio/gradio#10662

requirements.txt

@@ -1 +1,2 @@
 grote>=0.1.11
+pydantic<2.9.0